1. Field of the Invention
The invention relates to an annular exit die for an extrusion head for fabricating a tubular extrudate of thermoplastic material and having a tapered core and die casing ring cooperating to form an annular die gap or orifice. The cross-sectional dimension of the die gap is uniformly enlarged or diminished by moving the tapered die core axially over a prescribed distance and the cross-sectional dimension of the gap is further changed at prescribed points about its circumference by further axial movement of the die core.
2. Description of the Prior Art
Extrusion heads have been developed in which the die cross-section can be changed at prescribed points in a circumferential direction about the die gap in order to obtain a specific distribution of wall thickness in the hollow extrudate material. When larger hollow bodies, such as barrels are fabricated, the hollow extrudate is extruded into a segmented blow mold cavity. The lower end of the extrudate is pulled over blow pins which are spread apart before the mold segments begin to clamp. As a result of the pulling action and the elasticity of the extrudate material, the lower end of the extrudate has a varied thickness.
When the mold segments clamp together, the hollow extrudate is squeezed along the plane of the mold division on the top and on the bottom of the cavity and is welded. The hollow extrudate is then blown into the mold shape and contacts the inner surface of the mold. During the molding process, the hollow extrudate must traverse different stretch paths at different points along its circumference and at the upper and lower clamping points, i.e., at the mold separation steam. As a result of the extrudate stretching, the wall of the finished blown hollow body will have a thickness that varies as a function of extrudate material stretch. In cylindrical hollow blow molded bodies, these differences in wall thickness are particularly noticeable in the end walls which have a maximum thickness in the neighborhood of the mold separation seam and a minimum thickness in the zones which are circumferentially furthest away from the mold separation seam.
At the outer ends of the mold separation seam as they extend across the top and bottom of the mold the material almost contacts the inner contour of the mold, and thus undergoes a very little stretch. However, at the zones spaced 90.degree. from the mold separation seam, the material must be stretched over the entire radius of the hollow body to form the top and bottom of the hollow body and to come into contact with the interior side wall of the mold. In the blow molding process, blown air used to stretch the extrudate acts everywhere in the mold under the same pressure. Therefore the extrudate material does not flow from thick wall points where only a little stretching takes place to thin wall points where more pronounced stretching takes place.
For these reasons, the hollow extrudate must have sufficiently thick walls throughout such that the weakest points in the finished blown hollow body product will still have sufficient stability. This entails the use of more thermoplastic material than for a blown hollow body having uniform wall thickness. It also necessitates longer cooling times for the molding process and consequently entails production inefficiency.
The teachings of French Pat. No. 1,385,115 sought to solve the problem of changes of wall thickness in the hollow extrudate by changing the die gap during the extrusion process. The die core, which was movable in the axial direction with respect to the die casing, had an essentially cylindrical shape. Beginning at half its length, the die core had flattened sections along two opposite sides, which dropped down diagonally toward the inside and bottom of the die core. The cylinder wall disposed in the area between the flattened sections remained constant, so that the die gap in these areas could never change regardless of the setting of the die core.
The above device was suitable only for producing relatively small hollow bodies, since large hollow bodies require a capability for adjusting the extruding die to change the extrudate thickness along its circumference or along its length. For example, when extruding a long hollow extrudate in a vertical downward direction, the wall thickness of the hollow extrudate stretches under its own weight. To compensate for the extrudate stretching under its own weight the die gap is widened by appropriately changing the relative axial positions of the die core and the die casing. The reduction of wall extrudate material thickness due to stretching is accordingly balanced out by the addition of more material.
The above described device does not provide for increasing the die gap as a function of the length of extrudate material, since the die core has only the partial displacement capability at the flattened sections.
In order to vary die gap thickness without any deformation of die components, German GM No. 79 32 422 disclosed that the cross-section of the die opening can be partially changed at prescribed points by a positioning ring that is movable with respect to the extrusion head along the axial direction of the latter. If the end surface of the die casing in the circumferential direction is radially symmetrical, the horizontal lower end surface of the positioning ring forms an undulating, wave-like pattern. Conversely, if the horizontal end surface of the die casing forms an undulating pattern, the horizontal lower end surface of the positioning ring is radially symmetrical. At the points where the die opening crosses the separation seam of the blow mold, an undulation peak constricts the die gap cross-section corresponding to the setting of the positioning ring. Beginning at the area of the mold separation seam, this peak constantly decreases to a die gap circumferential angle of 90.degree.. From there, it steadily rises again to the top of the following undulation peak. The undulation wave trough bottoms leave open the full cross-section of the die gap opening.
With such a device, both the die core and the die casing ring, or respectively the positioning ring, had to be designed so that they were mutually adjustable in height. This meant that relatively complex displacement motions between the die core, die casing ring, and positioning ring had to occur during the extrusion process. This required a comparatively expensive component displacement mechanism. It is indeed possible to displace the die core without much difficulty. However, an additional displacement device is necessary for the die casing ring and/or the special positioning ring.